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BIG PHYSICS, BIG QUESTIONS –

Stellar ‘partner swaps’ yield cosmic explosions

By Maggie McKee

Stellar “partner swaps” may be responsible for some of the intense, momentary cosmic explosions called short gamma-ray bursts, a new study reports. It suggests explosions from these swaps may be observed more easily than those from the collision of stellar siblings, which may actually be more common.

GRBs are volleys of very-high-energy photons that can originate from any direction in the sky and come in two classes. “Long” bursts last from seconds to minutes and have been found to coincide with powerful supernovae, suggesting they form when some massive stars explode and their cores collapse into black holes.

But until recently “short” bursts – lasting just a split-second – have proved frustratingly elusive, disappearing without a trace before researchers could pinpoint or study them. Then in May 2005, NASA’s Swift space telescope spun around in less than a minute to catch the first X-ray afterglow from a short burst. That allowed astronomers to identify a general location for the burst, near a galaxy full of ancient stars.

About six short GRBs have been observed since, and they also tend to come from the outskirts of galaxies or from galaxies full of very old stars. The observations have lent support to the “coalescence model”, which suggests that short bursts occur during mergers between two stellar corpses called neutron stars, or between a neutron star and a black hole.

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Globular clusters

In this model, the two objects, unable to escape each other’s immense gravity, can nevertheless take billions of years to finally collide and merge. During this time, the pair may travel out of the galaxy in which they formed – which may explain the observed location of the bursts.

Previous theoretical work has suggested that most of the neutron stars in these mergers formed and evolved together. Now, researchers led by Jonathan Grindlay of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, says another type of merger may account for many short GRBs.

The team says the mergers could occur in dense swarms of ancient stars called globular clusters. The clusters teem with as many as 10,000 stars per cubic light year, and the overcrowding can cause stellar pairs to lose or swap their partners in complex interactions.

If two stellar pairs – each containing a neutron star – exchange partners, the resulting neutron star duo will eventually merge and produce a short GRB, says Grindlay. “Globular clusters are so closely packed that you have a lot of interactions,” he says. “It’s a natural way to make double neutron-star systems.”

Mismatched spins

The researchers calculated how often such mergers would occur by running computer simulations of globular clusters. They found that between 10% and 30% of short GRBs come from these stellar swaps.

But Grindlay says that although these mergers are less common than mergers from stellar siblings – they are seen more often. He argues that all of the short GRBs observed to date arose in regions where globular clusters are expected to lie.

These mergers may also be easier to observe as in these rather chaotic stellar swaps, the spins of the resulting neutron stars may be mismatched. This means the beams of radiation produced in the event may be wider than those produced by stellar siblings, whose spins may be aligned with each other.